So I just acquired a used pod. I really like it. But I have an issue with buzzing /humming through headphones or amp. The noise stops when I touch the pod or metal on my guitar. The noise isn't there with no guitar plugged in and if I turn the guitar down the noise goes away. I've tested two different guitars and both do it. However, neither of these guitars buzz on any other amp. Another interesting thing, when connected via USB it seems to ground it and there is no buzz. Could it be the power supply? The guy had replaced the original with a cheap Chinese power supply. Could that be it or should I look for a different problem?